I currently have a 2012 F150 5.0 (1st gen for this motor) I'm happy overall with its performance, bought the truck with 82k about to hit 120k, do a fair share of towing with it and I've had it loaded down pretty good. Recently took a hit on mpg but its tough to tell why as no two tanks of gas are the same and almost all have some towing/hauling on them.
I am always looking at new trucks and recently took a look at the 3.5 ecoboost. Until this I was set on another 5.0 (if I stuck with an F150) because the V8 is "simple" even with the 2021 cylinder deactivation, great opportunity to save fuel I told myself... In reality the 5.0 is on its 4th generation since it was introduced in 2011. power is up, I would hope reliability but with the requirement of new "features" like the cylinder deactivation its hard to tell. All vehicles today are very complex machines, the reality of mass production means that statistics are used to control quality and there are acceptable defects, I wouldn't be surprised if the motor heads mentioned above labeled as "defective" were because they very well could have been.
That said, the 3.5 ecoboost was introduced in 2011 as well and is only on its 2nd generation, from my recent research it seems they identified weak points on the first gen and addressed them with the second gen. There will always be opportunity for failures, I believe most defects should work themselves out well within the manufacturer warranty and that's why its still provided, I'm sure manufacturers would love to drop warranty coverage and save themselves $ on running an entire department but they know they'd spend as much or more taking calls on complaints or have to spend much much more to guarantee defect free products.
If I were to go buy an f150 today my first choice would be the 3.5 ecoboost, it would be best for my application, towing/hauling. I think the 5.0 would do fine as well so if I found a trim that had what I wanted with either engine I don't think I'd walk away based on either engine
my $0.02